$-\dfrac{1}{8} + \dfrac{7}{10} = {?}$
Solution: $ = - {\dfrac{1 \times 5}{8 \times 5}} + {\dfrac{7 \times 4}{10 \times 4}} $ $ = - {\dfrac{5}{40}} + {\dfrac{28}{40}} $ $ = - \dfrac{{5} + {28}}{40} $ $ = \dfrac{23}{40}$